The invention discloses an improved process for production of cross-linked rubber particles, which are suitable for the partial or complete substitution of conventional fillers, such as carbon black or silicic acid, in rubber vulcanisates.
The use of rubber gels in vulcanisates is described, for example, in U.S. Pat. No. 5,395,891 (BR-gels) or in EP-A 854 170 (SBR-gels).
The normal procedure for producing cross-linked rubber particles (also known as rubber gels or micro-gels or gels), consists in the conversion of polymer dispersions (polymer dispersed in water) especially the conversion of rubber dispersions (rubber latices), with a peroxide, whereby the peroxide is added to the rubber latex and converted at a relatively high temperature corresponding to its half life. In this context, the dispersed rubber is cross-linked. To avoid subsidiary reactions of the peroxide with oxygen, the conversion is implemented either under an inert-gas atmosphere or the reaction mix is inertised by repeated cycles of pressurisation and depressurisation with inert gas.
In U.S. Pat. No. 5,395,891, Examples: BR gel A1, after addition of dicumyl peroxide, the autoclave is pressurised with 8 bar nitrogen pressure. The reaction is carried out at 150xc2x0 C. Under these conditions, the pressure rises to 15.0 bar. Accordingly, a reactor must be selected which is capable of withstanding this pressure.
In EP-A 0 854 170, Examples 2, 3 and 4, after addition of the dicumyl peroxide, the reaction is pressurised with 5 bar nitrogen pressure and then depressurised. This procedure is repeated 3 times. The advantage of this procedure is that when the reaction is being implemented, maximum pressures of only 5 bar occur. However, the three-fold pressurisation and depressurisation with nitrogen is extremely troublesome and therefore costly.
The object was therefore to simplify the procedure in such a manner that, on one hand, the pressures arising in the reaction remain as low as possible, so that autoclaves certified for  less than 6 bar can be used, and on the other hand, so that the troublesome handling of nitrogen can be avoided.
The object is resolved by implementing the conversion of the rubber latices with organic peroxides without inertisation. Under these conditions, at 150xc2x0 C., maximum pressures of 5 bar occur. The rubber gels obtained with this procedure do not differ from rubber gels produced in accordance with the prior art, either with reference to their analytical data or with reference to their functional capability in rubber vulcanisates made from them.
The object of the present invention is therefore a process for production of cross-linked rubber particles by conversion of rubber latices with cross-linking agents, characterised in that the reaction mix is not inertised before the implementation of the cross-linking reaction.
As mentioned above, it is important for the process according to the invention that no measures for inertisation are carried out before the cross-linking reaction with the rubber latex to be treated with cross-linking agents. This means that the atmospheric oxygen is not removed by known measures for inertisation, such as rinsing and operating under nitrogen.
The cross-linking of rubber latices in accordance with the process of the invention in the presence of organic peroxides is otherwise implemented under the known and conventional conditions (U.S. Pat. No. 5,395,891; EP-A 0 854 170).
For the cross-linking of the rubbers, the normal cross-linking agents are used, such as organic peroxides, e.g. dicumyl peroxide, t-butylcumylperoxide, bis-t(butyl-peroxy-isopropyl)benzol, di-t-butyl peroxide, 2,5-dimethyl hexane-2,5-dihydro-peroxide, 2,5-dimethyl hexine-3,2,5-dihydroperoxide, dibenzoyl peroxide, bis-(2,4-dichloro-benzoyl) peroxide and/or t-butyl perbenzoate, and organic azo compounds, such as azo-bis-isobutyronitrile and/or azo-bis-cyclo hexane nitrile. The use of dicumyl peroxide is preferred.
The optimum temperature for implementation of the cross-linking is naturally dependent on the reactivity of the cross-linking agent, and this can be carried out at temperatures from room temperature (approx. 20xc2x0 C.) to approx. 180xc2x0 C., optionally under increased pressure (1 to 5 bar), (in this context see Houben-Weyl, Methoden der organischen Chemie, 4th edition, Volume 14/2, page 848).
The process in accordance with the invention is suitable for the cross-linking of aqueous rubber dispersions and also for rubber latices produced by emulsion polymerisation, for rubber dispersions obtained through re-dispersion of so-called solvent rubbers and for naturally occurring latices such as natural latex.
The particle diameters of the polymerisates used for cross-linking are within the range 5 to 10,000 nm. A range from 20 to 600 nm is preferred (DVN value in accordance with DIN 53 206).
The conversion of the cross-linking agents with the rubber latex and/or the rubber dispersion is recognisable in that the polymer density, the gel content and the glass transition temperature increase proportionally with the degree of cross-linking. By contrast, the swelling index declines as the cross-linking increases.
The glass transition temperatures of the rubbers used and cross-linked are measured by means of DSC (Differential Scanning Calorimetry) (e.g. Kalorimeter Pyris DSC-7 manufactured by Perkin-Elmer). For measuring the glass transition temperatures, 11.6xc2x10.3 mg substance are used in normal capsules. Two heating phases in each case from xe2x88x92100xc2x0 C. to +150xc2x0 C. at a heating rate of 20K/min and one cooling phase of 320K/min with nitrogen rinsing are implemented. The glass transition temperatures are measured during the second DSC heating phase. The glass transition temperatures are increased by cross-linking. The normal increase in glass transition temperature is by 5 to 50xc2x0 C.
The gel content and swelling index of the rubber gels obtained are determined with reference to a precipitated and dried polymer sample by swelling 250 mg of gel in 25 ml of a suitable solvent, e.g. toluene, for 24 hours while shaking. The gel is centrifuged off at 20,000 rpm, and the wet weight is determined; it is then dried at 70xc2x0 C. until a constant weight is achieved and weighed again. The gel content is calculated from the quantity of dried polymer relative to the polymer used and shown as a percentage by weight. The gel contents of the cross-linked rubber particles normally provide values  greater than 50 wt. %.
The swelling index [Swi] is calculated from the weight of the solvent-containing gel (after centrifugation at 20,000 rpm) and the weight of the dried gel as follows:   Swi  =            Weight      ⁢              xe2x80x83            ⁢      of      ⁢              xe2x80x83            ⁢      the      ⁢              xe2x80x83            ⁢      wet      ⁢              xe2x80x83            ⁢      gel              Dry      ⁢              xe2x80x83            ⁢      weight      ⁢              xe2x80x83            ⁢      of      ⁢              xe2x80x83            ⁢      the      ⁢              xe2x80x83            ⁢      gel      
The swelling indices of the cross-linked polymers (Swi) in toluene are 1 to 50, preferably 1 to 25.
If the latex is adequately stabilised during the cross-linking, e.g. with emulsifiers, the distribution of particle sizes hardly changes during the conversion. If the latex is not adequately stabilised, deposits may be baked onto the reactor wall during the conversion, stippling may occur or the entire reaction mix may even coagulate. One reason for latex instabilities during the reaction is, for example, that the pH declines during the reaction and as a result, emulsifiers based on carboxylic acid are rendered ineffective. In such cases, it may be helpful, for instance, to create an alkali reserve by adding an adequate quantity of alkali, or to keep the latex pH constant during the reaction by pumping in additional alkali. Another possibility for stabilising the latex is to add suitable emulsifiers either before or during the implementation of the reaction.
In accordance with the inventive process, rubber particles based on polybutadiene (BR), styrene-butadiene-copolymerisates (SBR), natural rubber (NR), nitrile rubber (NBR), polyisoprene (IR) and polychloroprene (CR) can be produced.
The cross-linked rubber particles produced according to the invention are stabilised in the normal manner, coagulated and dried. During isolation from the aqueous dispersion, they are either isolated in a pure form or after addition of an un-cross-linked rubber latex as a master batch. The production of master batches with natural rubber, with styrene/butadiene rubber, with nitrile rubber and with polychloroprene is conventional.
For use as polymeric fillers in rubbers and thermoplasts, the cross-linked rubber particles are compounded and vulcanised with the conventional mixing components.
The cross-linked rubber particles produced in accordance with the invention on the basis of the rubbers named can be used for the production of rubber vulcanisates, which can again be used for the production of an extremely diverse range of formed rubber bodies.